In wireless communications, a power amplifier is often required to produce a range of output power levels. For example, in a mobile telephone system, a base station may dictate the power level at which each mobile handset should transmit (based on factors such as the physical distance from the base station, for example). A critical performance metric for handset power amplifiers in this type of environment relates to efficiency, as defined as the ratio of the power delivered to the antenna to the power drawn by the power amplifier. Simple power amplifiers typically achieve their highest efficiency for their maximum output power, and their efficiency falls off significantly as the desired operating output power is reduced from this maximum (a condition referred to as back-off). To maintain high efficiency in back-off operation, it is beneficial that a power amplifier utilizes a segmented architecture, whereby multiple power amplifier segments can deliver power to the antenna. The number of power amplifier segments that are turned on at a given moment will depend upon the output power requirements at that moment. Segmented power amplifier architectures require a low-loss means to combine and deliver to the antenna the power output by one or more turned-on power amplifier segments, while other power amplifier segments are turned off.
The efficiency of power amplifiers is further degraded when certain design assumptions are not met in operation. One important design assumption is the antenna impedance, which is typically assumed to be 50 ohms. However, handset design and the environments in which handsets are used cause the antenna impedance to vary. Antenna impedance will change due to a variety of factors, such as changing handset geometry (e.g., closing a flip phone can change the antenna impedance) or a change in the region surrounding a handset (e.g., placing the phone next to one's head). As a result of these changes, the power amplifier is less efficient at coupling power into the antenna.
To accomplish impedance matching and maintain high efficiency, the optimal load impedance of the power amplifier must be varied to match the impedance of the antenna.
There is therefore a need to efficiently and adaptively combine the power output by multiple power amplifier segments, and to efficiently and adaptively match the power amplifier's optimal load impedance to the variable antenna impedance. Previously, PIN diodes and GaAs FET switches have been used to connect and disconnect segmented power amplifier sections in order to improve efficiency of operation in backed-off mode. Also, PIN diodes, GaAs FET and SOS FET switches have been used to adjust the reactance of the power amplifier output matching networks in order to improve coupling to the antenna when the antenna is detuned. However, the PIN diodes consume current even in the off state, and GaAs FET devices and SOS FET devices cause a high insertion loss. Consequently, the use of these structures can result in a loss in efficiency and/or undesirably large power consumption in the switches.
Aoki, I. et al., “Distributed Active Transformer—A New Power-Combining and Impedance-Transformation Technique,” IEEE Transactions on Microwave Theory and Techniques, Vol. 50:1, pp. 316-331 (January 2002), describes a distributed active transformer (DAT) structure for on-chip impedance matching and power combining. The solution uses push-pull amplifiers, ac virtual grounds, and magnetic coupling for series power combining. U.S. Pat. No. 6,856,199 entitled “Reconfigurable Distributed Active Transformers” also describes a DAT. An alternative power combiner configuration (i.e. transmission-line transformer) is described in Niknejad, A., et al., “Integrated circuit transmission-line transformer power combiner for millimeter-wave applications,” Electronics Letters, Vol. 43:5, (March 2007). U.S. Pat. No. 7,161,423 to Paul at al. also describes a power amplifier with multiple power amplifier segments that can be selectively disabled to provide different output power levels. However, the power amplifier segments that are turned off in back-off condition introduce an undesirable parasitic load on the circuit. Therefore, these solutions do not provide for efficient conversion of the power amplifier output impedance to match the antenna impedances in a back-off condition.
A power dividing/combining apparatus is also described in US Patent Application Publication No. 2008/0001684 entitled “Power Combiners Using Meta-Material Composite Right/Left Hand Transmission Line (CRLHTL) at Infinite Wavelength Frequency” and filed on May 3, 2007. Transmission lines in this reference are made of highly specialized materials that are used to create composite right/left handed lines that form a zero degree line. Tunnel-diode oscillators are connected directly to such lines to combine the power in-phase. Alternatively, zero degree transmission lines are used to implement stationary-wave resonators with oscillators loosely coupled to them. However this solution requires the use of very specialized materials manufactured in specialized processes. Furthermore, when all oscillators are not simultaneously turned on, the inactive portions of the transmission lines may present high parasitic loading to the entire circuit. The use of switches with this design would reduce efficiency and increase power consumption.
Consequently, power combiners with improved efficiency for impedance matching to the variable-impedance antenna are sought.